already at hand to care for a long period of unusual consumption. For practical purposes it will be sufficient generally to draw at such infrequent intervals from the emergency reserve.

In this discussion, calculation of the amount of filtered water storage space required, with a given total filter capacity, to meet the demand of periods of unusually high consumption, has been based upon the assumption that the filtered water storage will not be drawn down except at such times as the maximum rate of filtration cannot supply the draft. As it is impossible to recognize the existence of a critical period of high water consumption during its early stages, purification works should be operated in such manner as to hold in storage the maximum possible amount of filtered water immediately upon the commencement of either hot or cold weather. If this is not done, the emergency reserve of filtered water may have to be drawn upon.

It is immaterial, as far as the quantity of filtered water storage space required is concerned, whether all of this space is provided at the purification works or at some point or points on the distribution system. Possession of more than one reservoir for filtered water often promotes economy and increased operating efficiency.

A comparison of filtered water storage space and total filter capacity for a considerable number of filtration works is interesting. Table 17 gives these data. The net daily capacities of the plant. have been calculated also, upon the basis of the curves given on figure 11. From the results given in table 17, figure 12 has been prepared. This diagram shows the net daily capacities of the works in per cent of total daily filter capacity.

When considering individual plants listed here, it must be borne in mind that the local water consumption does not vary exactly as it did in the city whose records were used in deriving the net capacities of all the plants. The actual net capacity of the works in any of the cities mentioned will be greater or less than the figure given in the table, depending upon whether the periods of maximum water consumption are locally less or more severe and prolonged than in the control community. The records upon which this study is based probably are fairly representative. However this may be, when it is found, by the same measure, that the net capacities of seventy-seven water purification works range between 70 and 85 per cent, and most of them between 75 and 80 per cent, of the total

filter capacity, there is excellent ground for belief that the sustained yield of such plants in general is much less than their rating, and that their capacities have been fixed from somewhat similar, but perhaps not fully rational, considerations. Wide-spread deficiency in actual capacity becomes all the more apparent as the calculations have taken account of no reserve to meet accidents or other emergencies.

If most water purification works had the capacity they are supposed to possess, enlargement would not generally follow original installation so quickly.

CHAPTER XV

PUMPING STATION PRACTICE

Boilers

In the average steam-operated pumping station, large preventable waste of money often occurs in the boiler room. This comes from overlooking the fact that the boiler room is a manufacturing plant, producing from fuel, air and water a product, steam, by sensitive chemical and physical changes. As in every manufacturing plant, low cost of production can only be attained by preventing needless waste, and the search for and prevention of such wastes has been found by many water works managers to be an interesting as well as profitable undertaking.

Money is saved in a boiler plant by handling fuel and ashes at minimum expense, by burning the fuel as completely as practicable with a minimum quantity of air, because heating air not needed for combustion reduces the heat available for making steam, and by transferring the heat from the gases of combustion to the water (and to the steam in the case of superheaters) with minimum losses during transmission.

In carrying on these operations unremitting attention to safety is imperative, for any boiler in operation is potentially dangerous. Fortunately, by requiring boilers to be built in accordance with the Boiler Code of the American Society of Mechanical Engineers, as well as the boiler code of the State where they are to be used, and having them inspected by a responsible boiler insurance company, the good construction and safety of new boilers is readily assured. After they go into operation, it is desirable to have them similarly inspected internally at least once a year and externally twice a year.

Types of boilers. The type of the pressure parts of a boiler is the first feature about a boiler room to be considered in an investigation to ascertain the way to operate it most economically. In every type of boiler used in pumping stations a considerable proportion of the metal walls through which the heat of the hot gases is transmitted to the water is provided by tubes. If the hot gases pass through

the tubes the boiler is called "fire tube" and if water passes through the tubes the boiler is called "water tube."

Fire tube boilers. In fire tube boilers, the tubes are within a large drum or shell. With very few exceptions the fire tube boilers used in pumping stations are set with this drum horizontal, with the furnace at one end, the gases passing backward outside the shell to the rear of the shell where they enter the fire tubes and pass forward to the front, whence they are taken through breeching to the chimney. On account of this arrangement of the boiler in its setting, it is termed a "horizontal return tubular boiler."

Increase in the diameter of the shell or in the steam pressure requires proportional increase in the thickness of the shell plates and, as it is generally held that the danger of overheating the plates directly over the fire increases with the thickness of the plates, the size and working pressure of fire tube boilers are much below those of the average water tube boiler now built, except for locomotive and marine service. Today fire tube boilers for stationary use are rarely built for a working pressure exceeding 150 pounds or larger than 200 horse power in rating. In their proper field, which includes many small pumping stations, they are economical, if properly installed and operated, and a good, small set of such boilers costs much less than a comparable set of water tube boilers.

Water tube boilers. Water tube boilers are classified according to the position of the water tubes into vertical, inclined and horizontal types. In the last the tubes are not horizontal, but inclined at an angle of about 15° with the horizontal. A few water tubes boilers combine features of two types. By varying the arrangement of the tubes in many types of these boilers, without changing the surface area of the tubes, it is possible to provide either a boiler of the highest efficiency in absorbing heat, such as is desirable for operation under a steady load, or one of somewhat lower efficiency, but capable of quickly responding to a heavy increase in the load, such as is suitable for a station operating under fluctuating loads. While the two boilers will have the same horse power rating, the resistance to draft in the former will be greater than in the latter.

Nearly all the heating surface of water tube boilers is in the tubes, and it is practicable to use drums of much smaller diameter than those of fire tube boilers of equal rating. The tubes and drums are so connected that the entire assemblage of pressure parts can be

supported in a way leaving the parts free to expand and contract without placing any weight or strain on the masonry of the setting, which is very desirable. Fire tube boilers can be similarly supported, but quite often they are supported by the walls of the setting, in order to save the expense of independent steel supports.

Water content of boilers. Water tube boilers contain less water per square foot of heating surface than do fire tube boilers, and consequently it is easier to maintain the correct water level in the latter than in the former. Practically, this is unimportant if good firemen are employed, and it results in greater ease in forcing the boilers when a sudden peak load comes on the station.

The quantity of water per square foot of heating surface in boilers varies considerably with their design and size. A 250 horse power vertical water tube boiler will probably average about 1.4 cubic feet of water per square foot of heating surface, a horizontal longitudinal drum water tube boiler about 1.3 cubic feet, and an inclined water tube boiler about 1.25 cubic feet. Horizontal return tubular boilers of this size are rarely built, but two 125 horse power boilers will hold about 1.9 cubic feet of water per square foot of heating surface.

Efficiency of pressure parts. The purpose of the pressure parts of a boiler is to transmit to the water the heat radiated from the fire and carried by the furnace gases on their way to the chimney. Such a small proportion of the steam is generated by radiant heat in most boilers that it is usually omitted from any investigation of efficiency, although it should not be forgotten, and only the absorption of heat from the gases is considered. The heat absorption is the more effective as the opportunity for contact between all portions of the furnace gases and the heating surfaces of the boiler is increased.

In the horizontal return tubular boiler, the hot gases pass from the furnace and combustion chamber over a large part of the shell of the boiler to the back of the setting, and it is usually believed that only a small part of them comes into actual contact with the shell. At the back of the boiler, the heat of the gases tends to force a large part of them through the upper rows of fire tubes. This unequal distribution of the gases over the heating surface is responsible for a part of the difficulty encountered in attempting to operate such boilers much above rating. Various methods of improving these conditions have been tried and some of them have been found meritorious on test, but none has come into general use.